Dewatering aqueous suspensions of organic solids



July 5, 1966 c. P. PRlEslNG ETAL 3,259,570

DEWATERING AQUEOUS SUSPENSIONS OF ORGANIC SOLIDS Filed March 4, 1963 @uwQM,

www mah@ Y mime 5 wp@ m @M w@ r /U Wg 00 m5 United States Patent Olhce3,259,570 Patented July 5, 1966 3,259,570 DEWATERING AQUEOUS SUSPENSINSQF ORGANIC SGLIDS Charies P. Priesing and Stanley Mogelnicki, Midland,Mich., assignors to The Dow Chemical Company, Midland, Mich., acorporation of Deiaware Filed Mar. 4, 1963, Ser. No. 262,405 9 Claims.(Cl. 21d-53) The present invention concerns a novel process fordewatering aqueous suspensions which comprise `the suspended phase,agglomerated, hydrophilic organic solids. More particularly, theinvention is concerned with a process for conditioning sewage sludges toeffect rapid .and efficient dewatering thereof.

Heretofore, aqueous suspensions of agglomerated organic solids such assewage sludges have been dewatered by 'any one of, |or combination of,lseveral techniques involving compaction, rltration or flotationoperations. In compaction operations, ydispersed solids in a liquidsystem are concentrated or thickened with the .aid of gravity,centrifugal or direct physical forces. Filtration is perhaps the mostfrequently employed technique for resolving `a liquid suspension ofsolids into liquid and solid phases. Fundamentally, iiltration isaccomplished by presenting the liquid suspension to a septum or tiltermedia permeable t-o ythe liquid, but impermeable to the solids, underforces such that the liquid phase is caused to tlow .through the septumor tilter media. Sometimes the forces which cause the liquid to ilow arethe result of direct pressuriza-tion on the upstream side of the iilterand in other instances reduced pressures on the downstream side of the(filter. In some tiltration operations the force utilized is Isimplygravity act-ing upon the solid suspension to lbe filtered. The last ofthe aforementioned dewatering operations, i.e., flotation, involvesphysically decreasing the density of the :suspended phase such that itis caused to separate from the liquid .phase and rise toward the suriaceof the liquid.

Regardless of the dewatering technique used, aqueous suspensions ofpredominantly organic solids, such as sewage sludges, are most difficultto dewater due to the extremely hydrophilic and .amorphous nature of.the solids. Such suspensions are usually essentially non-free settlingat concentration-s above about 0.1 Ipercent by weight. That is, littleor no densication occurs under gravity without further mechanical orchemical conditioning. Illustrative of such essentially non-freesettling suspensions of organic .solids :are the raw, digested andactivated municipal sewage sludges. In order to dewater such .sludges byvacuum sliltration, it is frequently necessary to employ as much as -50to 2G() pounds per ton of dispersed organic solids of inorganicadditaments such as lime and ferric chloride. While `cationic organicpolyelectrolytes, especially water-soluble, nitrogenous polymers, can beemployed to effectively condition the sewage sludges for tiltration, the:amounts of the cationic polymers required for this purpose render thetreatment economically prohibitive for many operations. A referencewhich describes a process -for `dewatering sewage sludges with a numberof cationic organic polymers is United States Patent 3,014,896.

It would be desirable, Iand it is an object of the present invention, toprovide a novel process to promote more eiiicient and rapid dewateringof essentially non-free settling, .aqueous suspensions of predominantlyhyd-rphilic organic solids. In particular, it is fan object to prov-idea novel process lrelating to the dewatering of sewage sludges by anyIone of the dewatering techniques involving compaction, iiltration `orflotation principles. A still further object is to provide a process fordewater- Cil ing sewage sludges requiring small amounts of chemicalreagents. Other objects and `beneiits will .become apparent herona-literAas Ithe invention is more fully described.

In the drawing, solid line curves graphically depict result-s achievedin tiiltering 1a digested municipal sewage sludge i-n Iaccordance rwithseveral representative embodiments of the invention. A broken line curvedepicts a control operation which forms .a basis for comparing theinvention with the prior art.

In particular, lthe present invention provides a novel process fordewatering liquid ,aqueous suspensions of agglomerated organic solids,which solids are amorphous and hydrophilic in natu-re and at aconcentration whereby the solids are essentially non-free settling.While it is known that such suspensions :can be conditioned foriiltration operations by the 'addition thereto of water-soluble, cati-`on-ic organic polymers (cationic polyelectrolytes), it has now beendiscove-red that the .addition t-o .the suspension, with mixing, of .adilute aqueous solution of a high molecular weight, water-soluble,ani-onic polymer (anionic polyelectrolyte) prior to the Iaddition of thecationic polyelectrolyte is most advantageous. Among the advantagesobtained are more rapid dewatering of the suspension with greatlyenhanced recovery of the organic solids from the Iliquid phase. Theagitation employed to accomplish the required mixing is suiiicient touniformly distribute the polymer throughout :the suspension but neitherso hard nor so long as .to lsignificantly break down the existingagglomerates. Anionic polyelect-rolytes contemplated herein arewater-soluble polymers having polyalkane backbones characterized 4as:containing a plurality of at least one .substituent ionic functionalgroup selected from the group consisting of carboxylic, sulfonic, alkalimetal carboxylate iand :alkali metal sulfonate groups.

The above conditioning process for dewatering operations is especiallyeffective if the total dilution of the suspension )with water ismaintained Within the range from about 0.01(S) to 1.0(S) volumes ofwater per volume lof sludge. In the Iforegoing S equals the solids, on.a dry basis, :in parts per parts of the liqurid suspens-ion.Suspensions contemplated herein are liquid in character and contain iatleast .about 0.1 part solids on a dry basis per 100 parts of thesuspension. Often they contain from 1 to l5 parts lof such solids per100 parts of liquid suspension. Higher yamounts up to 25 parts per 100parts are possible but rare in sewage sludges prior to dewatering.

The terminology dry basis in specifying solids contents herein hasreference to the percent by weight solids determined by evaporating theWater from a predetermined aliquot of the suspension at a moderatetemperature of about C. While this determination will include soluble aswell as insoluble constituents of the suspension, the soluble fractionis small as compared to the insoluble solids, i.e., suspended, fraction.Thus, treatments speciiied herein on the basis of suspension solidscomputed on a dry basis are essentially controlled by the insolubleportion ofthe suspension.

Typical suspensions that can be conditioned and dewatered according tothe invention include the various types of liquid, i.e., iiuid,municipal Waste sludges, eg., raw, activated (obtained by aerobicbioligical oxidation `of raw sewage), and digested (obtained byanaerobic biological oxidation of raw sewage) sewage sludges. Theprocess is also well adapted for rapid dewatering of most liquid sewagesludges of agglomerated organic solids encountered in the waste disposalsystems for cannery, textile, paper, meat packing, vegetable canning,fruit canning, petroleum reiining, dry and chemical industrial wastes,inclusive of raw, activated and digested sludge forms thereof. Generallythe conditioning process of the invention is applicable to non-freesettling sludges of hydrophilic organic solids, which sludges arecharacterized by a net negative charge as determined by electrophoreticmobility measurements. In other terms, the sludges affected are thosehaving a negative zeta potential.

In addition to the advantages of achieving rapid dewatering and bettersolids removal, sludges conditioned in accordance with the invention arecharacterized by exceptional stability against mechanical agitation. Astill further advantage is the substantial reduction in the amount of awater-soluble, cationic, nitrogenous polymer required to effect eicientdewatering.

The latter advantage is most surprising in view of the fact that thehigh molecular weight, anionic polyelectrolytes have little or nobeneficial effect on the dewatering characteristics of sewage sludgeswhen employed as the sole treating agent.

The anionic polyelectrolytes employed in accordance with .the presentinvention are high molecular weight, synthetic, organic polymerscharacterized as consisting of linear carbon to carbon chains(polyalkanes) obtained by vinyl polymerizing one or more vinylidenemonomers bearing a carboxylic or sulfonic acid group, or an alkali metalsalt of such acidic groups. Such polymers are homopolymers of theseanionic functional monomers or interpolymers thereof with other suitablevinylidene monomers capable of undergoing vinyl polymerization. In suchinterpolymers, it is essential that sufficient carboxylic or sulfonicacid bearing monomers, or salts thereof, or such monomers with othermonomers containing watersolubilizing groups be present in the nishedpolymer to render it water-soluble. Usually, a minimum of at least about50 mole percent of the combined monomer moieties should bearWater-solubilizing groups in order to insure that the resulting polymeris properly water-soluble. Also, it is essential that a minimum Iofabout 4 mole percent of the monomer moieties combined in the finishedpolymer should bear an anionic substituent of the specified class ofsulfonic, sulfonate, carboxylic and carboxylic groups.

By high molecular weight, as applied tto the above anionic polymericagents, is meant an average molecular weight of at least about 0.5million as determined by light scattering measurements. The termWater-soluble means dispersible in water to provide a Visuallycontinuous dispersion infinitely dilutable with water.

A preferred species of anionic polyelectrolyte for use in accordancewith the present invention is a high molecular Weight homopolymer of analkali metal salt of vinyl benzene sulfonic acid. Similar polymerscorrespond to Water-soluble copolymers prepared by the vinylpolymerization of a major proportion of vinyl benzene sulfonic acid, oran alkali metal salt thereof, and a minor proportion of a monomercopolymerizable therewith such as acrylamide, methacrylamide,acrylonitrile, methacrylonitrile, styrene, vinyl toluene, methylacrylate and the like. For good results, such polymeric agents shouldhave a molecular weight of at least 1,000,000 and more preferably of a-tleast 4,000,000. Further improved results are obtained with even highermolecular weights, provided the polymer remains effectivelywater-soluble.

Another variety of anionic polyelectroltye suitable for use inaccordance with the invention is constituted by high molecular weight,water-soluble copolymers of styrene and maleic anhydride. Suchcopolymers are generally employed in the form of the alkali metal saltsthereof, preferably the sodium salt, although the acid form obtained byacid hydrolysis of the anhydride rings may also be employed. In Vorderto obtain the advantages of the invention, it is necessary that thestyrene maleic anhydride copolymers have molecular weights of at least1,000,000 and preferably of at least 4,000,000.

Anionic acrylic polymers that can be employed in fthe present inventionare water-soluble, substantially linear polymers of high molecularweight obtained by the vinyl polymerization of acrylic acid, methacrylicacid, sulfoethyl acrylate, carboxyethyl acrylate or water-soluble saltsof the foregoing acidic monomers or by copolymerization of the acidicmonomers, or alkali metal salts thereof, with suitable amounts up toabout 96 mole percent of other vinyl monomers such as acrylamide andmethacrylamide.

Other suitable anionic acrylic polymers are obtained as the vinylpolymerization products of acrylamide or methacrylamide, or as thecopolymerization products of such monomers with other suitable monomersincluding, for example, acryl-onitrile, methacrylonitrile, lower alkylesters of the acrylic acids, vinyl alkyl ethers and the likemonoethylenically unsaturated compounds. During the polymerizationreaction or thereafter with appropriate treatments, carboxamine moietiesof the polymer are hydrolyzed to provide a desired number of anioniccarboxylic or carboxylate groups. Usually, such copolymers are preparedin reaction mixtures containing at least about mole percent of theacrylamide or methacrylamide and not more than about 15 mole percent ofthe other monomers.

Anionic polymeric agents can also be produced by hydrolysis of preformednon-ionic polymers. For example, vinyl polymerized forms ofacrylonitrile or methacrylonitrile can be hydrolyzed by reacting themwith an aqueous metal -hydroxide solution to convert the nitrile groupsto the corresponding alkali metal carboxylate groups. Similarly,polymers or copolymers of alkyl esters of un* saturated acids can besaponied with an alkali metal hydroxide to convert ester groups toalkali metal carboxylate groups.

Cationic polyelectrolytes used in the invention are water-soluble,synthetic, organic polymers characterized as containing in or attachedalong the polymeric chain a plurality of amino, imino or quaternaryammonium groups. These nitrogenous polymers are generally recognized ashaving a beneficial effect on the lterability of the aforedescribedsewage sludges when properly applied thereto. Unlike the anionicpolyelectrolyte employed herein, the cationic1 polyelectrolytes have nocritical molecular limitations. It is preferred, however, to operatewith higher molecular weight polymers.

Representative of such cationic, nitrogenous polymers are homopolymersand water-soluble copolymers of one or more monomers such asN-vinylpyridine and substituted derivatives thereof; mono, di, ortrialkylammonium salts, eg., vinyl benzyl trimethylammonium chloride;allylamine and `N-alkyl substituted derivatives thereof, aminoethylacrylate hydrochloride or aminoethyl methacrylate hydrochloride and, ingeneral, any ammonium or substituted ammonium alkyl acrylate ormethacrylate such as N-methylof N,Ndimethylaminoalkyl acrylate ormethacrylate, wherein the the alkyl groups contain 2 or 3 carbons, orthe like. Other suitable cationic, nitrogenous polymers are obtainedwhen a nitrogen containing monomer is copolymerized with one or moreother monoethylenically unsaturated monomers capable of undergoing vinylpolymerization, provided that the resulting copolymer is water-solubleand essentially free of anionic substituents. In such copolymers, atleast about 5 mole percent, preferably at least l0 mole percent, of themonomers combined in the copolymer should be cationic, nitrogencontaining monomers. Suitable monoethylenically unsaturated monomers tobe copolymerized with these cationic monomers include acrylamide,methacrylamide, acrylonitrile, the lower alkyl esters of acrylic andmethacrylic acids, vinyl methyl ether, N-vinyl oxazolidinone, N-vinylpyrrolidinone and the like. It should be noted that for the purposes ofpreparing cationic polymers for the present invention, the amide groups,such as in acrylamide, and nitrle groups, such as in acrylonitrilc, arenot sufficiently cationic. However, monomers containing these groups canbe employed in conjunction with other nitrogenous, cationic monomers toprovide effective cationic polymers.

When the comonomers employed in the preparation of cationic organicpolymers embody water-solubilizing groups, such comonomers may bepresent in amounts up to 95 mole percent of the combined moieties in theiinished cationic, nitrogenous polymer. When hydrophobic comonomers suchas the alkyl esters of acrylic and methacrylic acids or styrene areinterpolymerized with the cationic monomers, it is generally required,in order to produce a water-soluble polymer, that at least about 60 molepercent of the combined monomer moieties be hydrophilic, i.e., bearingwater-solubilizing groups.

A preferred class of cationic polymers is represented by water-solublepolyethylenimines of high molecular weight, substitutedpolyethylenimines and the mineral acid and quaternary ammonium saltsthereof. Some polyethylenimines suitable for use in the invention can berepresented by the formula:

wherein R is hydrogen or methyl and n has a value of at least 400,preferably of at least 2,000. Desirable polyethylenimines can be-characterized by the viscosity of aqueous solutions thereof. Thus,water-soluble polyethylenimines having viscosities of at least about 0.8centistoke and preferably at least 2 centistokes for an aqueous, onepercent by weight solution thereof are preferred cationic polymers.Also, the various salts of these polymers such as the interreactionproducts thereof with hydrochloric, sulfuric, phosphoric, carbonio andacetic acids are efficient cationic polymers.

The vinyl benzyl quaternary ammonium compounds preferred for use in thepresent invention are the homopolymers of vinyl benzyl quaternaryammonium salts or copolymers of a vinyl benzyl `quaternary ammoniumcompound with arcrylamide, methacrylamide or Watersoluble N-alkylsubstituted derivatives thereof. Said copolymers may contain inpolymerized form a very small amount up to about 95 mole percent of theacrylic comonomer. These polymers and copolymers contain a plurality ofgroups having the following structure:

CH2CH R onf-ira X- wherein each R represents a lower alkyl radicalcontaining from l to 4 carbon atoms, inclusive, and X represents ananion such yas hydroxyl, chloride, bromide, iodide, hydrosulfate,dihydrophosphate or the like. The preferred vinyl benzyl lquaternaryammonium polymers and Vcopolymers are those having a viscosity of atleast 1.2 centipoises for a 0.5 percent by weight solution thereof in anaqueous 4 percent by weight sodium chloride solution at 25 C.

Certain of the foregoing cationic or anionic polymers as well as otherrelated polymers operable in the invention can be prepared bymodification of preformed polymers. Thus, for example, a linear highpolymer of a vinylaryl hydrocarbon, such as styrene, vinyl toluene,a-methylstyrene, vinylxylene or the like can be sulfonated undercontrolled conditions to produce a water-soluble, substantially linearpolymer sulfonate useful as an anionic polyelectrolyte in the invention.Alternatively, such a polyvinylaryl hydrocarbon can be chloromethylated,for example, by reaction with formaldehyde and hydrogen chloride, andthe resulting chloromethylatcd polymer can be reacted with a tertiaryamine to produce a cationic polymer containing a plurality ofpolymerized vinyl benzyl quaternary ammonium moieties. Similarly, ahomopolymer of a vinyl benzyl halide can be reacted with a tertiaryalkyl amine to Iproduce the corresponding cationic quaternary ammoniumsubstituted polymer.

v the agglomerated organic solids.

To carry out the present novel conditioning process in lconnection withcompaction, filtration or otation processes, an aqueous suspension ofagglomerated, essentially non-free settling organic solids, e.g., amunicipal raw sewage sludge, is treated with a small but effectiveamount of a water-soluble, high molecular weight anionic polyelectrolyteas hereinbefore described. The treatment is accomplished by adding thepolymer in the form of a dilute aqueous solution to the sludge with mildagitation of the sludge. The solution added is generally made up toprovide a predetermined desired polymer dosage at a given dilutionlevel. Thus, the polymer solution may cont-ain anywhere from as littleas about 0.001 percent up to as much as 5 percent by weight polymersolids. While neither of these limits is critical, the lower limitrepresents a practical dilution limit and the upper limit represents anapproximate lphysical limit for the convenient and practical handling ofthe high molecular weight anionic polymer solutions which, at or beforethis concentration level, become too viscous for convenientmanipulation. The total dosage of the anionic polymer used will varysomewhat according to the chemical nature and molecular weight of thepolymer, but in general the amount employed is small in relation to theamount of cationic organic polymer used. For most operations the amountof the high molecular Weight anionic polymer used will Vary within therange from about 0.01 to l0 pounds of the polymer per ton of solids on adry basis. For cornpaction operations it is preferred to employ at leastabout 0.5 and up to as much as l0 pounds of the anionic polymer per tonof solids on a dry basis. Filtration operations are unique in thatcomp-arably small amounts of the anionic polymer produce signicantresults. Illustratively, at least about 0.1 and no more than 2.5 poundsof the polymer are employed per ton of solids.

Agitation of the aqueous dispersion during the addition of the anionicpolyelectrolyte is accomplished by Iany convenient agitating means.lExamples of suitable means inclu-de large slowly rotating paddles,eifervescing gases and moderately turbulent iiow in conduits which maycontain baies to increase agitation at low llow rates. In any event theagitating means used are designed to provide homogeneous incorporationof the high molecular weight anionic polymer into the aqueous suspensionof While it is preferred to avoid significant destruction of theexisting agglomerates, obtaining an intimate and uniform admixture ofthe polymer with the suspension is more important than avoiding somedestruction of the existing agglomerates. Thus, the extent of or controlof agitation in this step is less important than in the following stepwhereby the cationic organic polymer is added to the suspension.

The cationic organic polyelectrolytes, -as specified for employmentherein, are likewise added to the aqueous suspension in the form of adilute solution which may contain as little as 0.001 percent up to asmuch as 30 or 40 percent polymer solids depending upon the concentrationlevel at which the polymer solution becomes inefficient for applicationpurposes due to excessive viscosity. This polymer like the anionicpolyelectrolyte is -added to the aqueous dispersion with mild agitationwhich may be produced by any one of the above-described means, orsimilar means, whereby the cationic polymer is intimately distributedthroughout the suspended organic solids. These solids should remain in,or result in, an essentially agglomerated condition during the mixingwith the addition of the cationic polymer. While the ultimatedew-atering results will vary according to the exact chemical nature of4the cationic polymer and its molecular weight, good to excellentresults are usually achieved with the employment of from about 0.5 toabout pounds of the organic cationic polymer per ton of solids on a -drybasis. Frequently the `amount employed is within the range from -about 1to 10` pounds of the cationic polymer per ton of solids.

While it is implicit in the foregoing that Water of diludetention time.

7. tion will be incorpora-ted into the aqueous dispersion of organicsolids Ialong with the ltreating polymers, care should be exercised incontrol-ling this parameter so as to maintain `the number of volumes ofwater added per volume of sludge treated Within the specified range,i.e., 0.01(S) tto 1.0(S). This dilution water can be added as .thesolution mediu-m for the polymers or in yone or more `other separateoperational steps. The optimum total dilution in lany one instance willbe in par-t dependent upon the type of sludge being treated and secondlyupon the concentration of organic solids in that sludge.

A further process parameter, which should ybe lcontrol-led for bestresults in .the practice lof the invention is lthe rat-io of `the weightof fanionic polymer to the Weight of cationic onganic polymer employed.An overall range for nthis :ra-tio .is from about 1:1 to 1:20. Inconditioning sewage sludges for compaction oper-ations, rthis ratio willlie within the range from about 1:1 to 1:5. In filtration .operationsthe ratio is usually .Within `the range iirom 1:3 to 1:20. As -a generalIrule the amount of anionic polymer required `can be reduced, and .thusthe ratio of anionic polymer -to cationic polymer decreased through ltheemployment of milder yagitation during .the addition of the polymers.Illustratively, severe agitation whereby the initial agglomerates oforganic solids are significantly lbroken down, may result in theemployment of la 1:1 :ratio of yanionic polymer to cationic polymer.Milder agita-tion may decrease athis ratio yto about 1:15.

In one particular embodiment of this invention, `the point of additionto ythe suspension for the anionic polymer is remote, rather thanproximate, to the point of addition for lthe cationic polymer. Forinstance, Ithe anionic polymer is sometimes conveniently incorporatedinto the suspension .during a prior sedimentation or cla-nification:step whereby a waste stream is clarified and the solids iunderiiowforms -the non-free settling sludges which are of concern herein. Duringthe clarification or sedimentation :operation the anionic polymer isadded to the waste stream in yan amount within the range from 0.1 to 2.5pounds per lton of solids on la dry basis. Intimate mixing of @thepolymer with the solids, which form the sludge, readily occurs duringthe settling of the solids. Subsequently, at -a point more proximate tothe dewatering device, the cationic organic polymer is added to theanionic polymer treated sludge with proper agitation to provide `asludge conditioned for efiicient idewatering.

It will be recognized that other procedural embodiments for carrying out.the invention Aare possible. Thus, the dual polymer .treatment of ytheinvention can be utilized separate-ly or conjunctively with theemployment of other chemical :treating systems which may utilize alum,ferrie chloride or lime. Further, idewatering processes described hereincan be augmented through the employment of biological and mechanicalsludge .treating techniques.

While the foregoing description of the conditioning process of theinvention is generally applicable tto all types of dewatering operationsfor separating suspensions of essentially non-free settling, organicsolids such as compaction, filtra-tion and oatation operations, each of,these is a distinct process in itself. Thus, completion of thedei/watering process after accomplishing the dual polymer .treatmentwill involve parameters unique to rthe particular separatory techniqueutilized.

Illustratively, in .the first of .the aforementioned dewateringoperations, ie., dew-atering lby compaction or thickening, thedewatefring technique utilizes prolonged detention .or residence timesfor the polymer treated suspension in an essentially quiescent zone. Forinstance, sludge ithickener-s, wherein compaction according .to theinvention is most often carried out, are generally designed withcross-sectional areas, and for hydraulic underflow Arates, such that thesolids detention time within [the thickening unit Iis increased incomparison to the liquid This is in direct contrast to primaryelarifiers, which loperate lon the principle of Isedimentation lby`receiving primary waste and producing an runderfiow suspension ofagglomerated solids .and .an .overflow of a partially clarified liquid.In such clarifiers, the solids detention times are usually short incomparison to .the hydraulic detention time-s.

The residence ytime for effective thickening according .to ytheinvention `will vary depending upon the extent of compaction desired and`the amounts of polymers employed .to condition the suspension.Generally, detention times vof at least 30 minutes are required. Often,however, `detention times Nvill extend `to .as much as 4 to 12 ihours.Suspensions such as sewage sludges which are not .treated in yaccordancewith the invention will often require detention -tirnes from 12 `to 24hours. It is manifest from .the comparatively more rapid thickeningrates achieved in accordance with Ithe invention that the possibility ofsepticity in the thickened solids is significantly reduced since :thellat-ter condition is essentially a function of time. By avoidingsepticity more complete separation `of the solids from `the liquid phaseand a denser thickened solids phase are possible.

One of the process variables involved in compaction operations is therequirement that the suspension be diluted with water. For best results,the solids content should be within the range from about 0.1 to about 5percent by weight of the suspension. It is desirable t0 add the requireddilution water as the dispersing medium for the treating polymers. Uponincorporation of the treating polymers and necessary dilution water toadjust the solids content to a desired level, the conditioned suspensionis passed into a quiescent, detention zone, eg., sludge thickening unit.

Initially the polymer conditioned agglomerated organic solids undergosubsidence in the thickening unit at a constant rate. Subsequently, theconditioned solids undergo thickening at a decreased rate during whichperiod the solid agglomerates are being deformed and compressed.Compression of the solids during this latter stage is aided by theemployment of slowly rotating fakes and other mildly agitating means.Upon completion of the deformation of the agglomerates, a thickened andsubstantially dewatered sludge is obtained.

It is interesting to note that once compacted, the polymer treatedorganic solids obtained in this manner are not readily dewatered byfiltration. Thus, if the compacted suspension is to be filtered, it isnecessary to again treat the compacted organic solids in accordance withthe invention.

To accomplish filtration of an aqueous suspension of agglomeratedorganic solids, polymer conditioning is carried out as described abovebut the conditioned suspension should be filtered as soon as isconvenient after completion of the treatment with polymers. Anydetention of the suspension in a quiescent zone should be kept at aminimum. For best results it is desirable to filter conditionedsuspensions having solids concentrations of at least about 1 but notmore than about 15 percent by weight of the suspension. Manifestly,since filter yield is a function of solids loading per lter unit areaper unit of time, it is desirable to employ high solids concentrationsin the filter feed. However, for best results some dilution of thesuspension to be filtered is desirable. The degree of dilution to beused is primarily a function of the solids concentration according tothe aforedescribed formula for dilution. In general, the solidsconcentrations employed in filtering suspensions of organic solids arehigher than solids concentrations in suspensions which are to besubjected to compaction. For instance, a digested municipal sewagesludge which ordinarily contains about 11 percent by weight solidsv on adry basis is usually filtered better at an 8 percent by weight solidsconcentration than at either 4 or 12 percent solids. In a compactionoperation, however, the same sludge should be diluted to about 0.5percent by weight solids.

It should be recognized that the organic solid phases to be ltered inaccordance with the invention described herein may, due to their extremehydrophilicity contain a substantial amount of imbibed water. Completewater removal is nearly impossible, but filter cakes obtained byfiltering suspensions treated in accordance with the invention willordinarily contain substantially smaller amounts of water than filtercakes obtained by methods of the prior art.

In iiotation dewatering operations, the suspension of l organic solidstreated with the dual polymer system is subjected to compressed air.Typical pressures used are within the range from about 2O to about 60pounds per square inch gauge. The compressed air is maintained over thesuspension for a period sufficient to permit saturation of the liquid byair. Upon release of the air pressure above the liquid suspension, thedissolved air coming out of solution causes the solids to rise towardthe surface of the liquid due to buoyant forces. Flotation operationsare similar to compaction operations by gravity in that the former islikewise best carried out on suspension having from about 0.1 to about 5percent by weight solids on a dry basis.

The following examples illustrate the general dual polymer conditioningprocess of the invention and specific instances of its adaptation tofiltration and compaction dewatering processes for various suspensionsof organic solids.

EXAMPLE 1 The following operations were designed to demonstratefiltering efliciencies realized by the timely addition of small amountsof an anionic polyelectrolyte to a digested municipal sewage sludgecontaining about 10 percent by weight organic solids, which sludge isconditioned for filtration by incorporating a cationic organic polymertherein.

The test procedure involved measuring 100 milliliters of the sludge intoa 250 milliliter beaker. To this sludge was added 25 milliliters ofdilution water having dissolved 40 pouring the treated sludge back andforth between 25() D milliliter beakers.

Whatman filter paper. The lter was seated in a vacuum flask wherein avacuum of l5 to 29 inches of mercury was maintained throughout thedewatering operation. Dewatering of the treated sludge was carried outfor two 5 minutes, as determined from the time of initial vacuumapplication, and the volume of filtrate collected during this period wasmeasured. These results are graphically depicted in the accompanyingdrawing wherein the volume of ltrate collected during the specified twominute dewatering time is plotted as a function of the cationic polymerconcentration, with each curve having as -a constant parameter thedosage of the high molecular weight, ranionic polyelectrolyte used. Asis evidenced by these curves, very small dosages of the 'anionic polymerhave a very pronounced influence upon the dewaterability of the sludge.Such anionic polymers employed as the sole treating agent, however, havelittle or no effect and may actually reduce the dewaterability of thesludge.

EXAMPLE 2 Another series of filtration experiments were conducted todemonstrate the effect of dilution, as well yas the employment of thedual polymer treatment in accordance with the invention to dewatersewage Sludge. The experiments involved treating a series of 113milliliter aliquots of a digested municipal sewage sludge containing 16grams of solids on a dry basis. Five dilution levels were used with atotal of 2 aliquots at each dilution level. One of these aliquots was.treated with a water-soluble polyethylenimine at a dosage of 1 pound ofpolymer per ton of solids on a dry basis. A second aliquot, at eachdilution level, was given an additional treatment prior to theincorporation of the polyethylenimine with a dosage of l pound per tonof solids, on a dry basis, of a high molecular weight sodium polystyrenesulfonate. Each of these polymers was separately mixed with the sludgeas in Example l.

The polymer treated aliquots were then dew-atered in a Buchner funnelseated in a vacuum flask to which a vacuum varying from l5 to 29 inchesof mercury was applied. Volumes of filtrate accumulated at several timeintervals were observed and recorded.

The results of these dilution experiments are reported in the followingtable. The volume of filtrate is reported in terms of the net effectiveremoval of water from the sludge suspension. That is, the positivevalues reported show net removal of water. Negative values indicate thevolume of water added by dilution which remains in the system. Runs 1,3, 5 and 7 are for sludge aliquots treated only with thepolyethylenimine. Runs 2, 4, 6 and 8 are for sludge aliquots treatedwith the anionic polymer followed bythe cationic polymer.

Table 1 Net Water Filtered at Specified Time Intervals Sludge DilutionPercent Run Aliquot Water Solids in (ml.) (ml.) Diluted 0 20 40 60 120180 240 300 360 Sludge Sec. Sec. Sec. Sec. Sec. See. Sec. Sec. Sec.

A predetermined dosage of a cationic organic polymer, EXAMPLE 3 i.e., awater-soluble polyvinylbenzyltrimethylammonium chloride, was thenincorporated into the sludge with an additional 25 milliliters ofdilution water. Mixing of the cationic polymer into the sludge wasachieved by pouring the sludge from beaker to beaker four times asdescribed above.

The treated sludge was then poured into a 12.5 centi- In furtheroperations to test other combinations of polymer treatments, sewagesludges containing different amounts of suspended solids were dividedinto 100 milli- In control establishing experiments, ml. aliquots meterBuchner funnel containing two layers of No. 1 75 of the sewage sludgewere diluted to specied volumes and filtered. Certain diluted sludgevolumes were also treated individually with either an anionic polymer ora cationic organic polymer. The diluted and polymer treated sludgealiquots were then poured gently four times tained from such filtrationoperations for various sewage sludges from the sewage Itreatment plantof a small Midwestern city are set forth in the following table. Alsoincluded in the table for the purpose of comparison are from one beakerto another and thereafter into a 12.5 runs made utilizing conventionalflocculants such as fercentimeter Buchner funnel wherein the sludgeswere deric chloride and lime separately and conjunctively with wateredfor two minutes according to the procedure of the dual polymeric systemof the invention. Example l. At the conclusion of the dewatering period,v the vacuum was broken, the filter cake, if any, observed Table2.-Fzltratl0n of sewage sludges Do c of- Dose of- Volume s Dry Dilutedof Fil- Run Sludge Type Solids Sludge trate in Lbs./ Lbs./ (gms.) (ml.)2 Min. Anionic Polymer Ton Dry Cationic Polymer Ton Dry (ml.)

Solids Solids 1 (control) Anaerobically digested 10 15o 2o 2(centrol).-. dO 10 150 24 3 mit 2'3 iig it t fffo) 1.5 150 13o 1.1 o 130SPSS(6M) 2. .8 150 130 sPsStcM) 2 .5 150 13o sPsseiM) 2. .625 150 13o2.0 150 11o sPss(6M) 1.0 150 12s sPSS(eM) 2.0 140 11s SPSS(6M) 2. 0 140124 sPss(eM) 4. 0 16o 123 sPSSM) fg 160 13o SPSSu) no 150 114 17sPsstsM) 5(1)'0 15o 115 18 (control) 1.0 150 121 sPSSteM) .5 15o 124 20(control).- PAAm 5-- 150 4() 21 PAAm-. .5 150 122 22 (control) 180 35 23(conrolL- 2. 0 l 24 1.0 18o 144 2e DMAEM 1. 0 iso 158 27 (contrel) 0.180 30 23 DMAEM 1. 0 180 156 29 SSME 1.0 18o 15s 30 (control) 3. 180 15o31 PAAm 30) 2.0 180 15s a2 SPSS(6M) 2.0 180 15s 33 (control) 220 45 34(control). SPSS(6M) 220 45 35 (control) 5 220 150 3e SPSS 6M 1.5 PE1@ 5220 175 37 sPSSteM) (added 1ast) 1.5 PEI@ (added first) 5 220 65Footnotes to Table 2; as employed in the present specification:

1 VBT refers to a homopolymer of vinylbenzyl trimethylarnmoniumchloride. A 0.4 percent by weight solution of this polymer ln 2 percentaqgieons sodium chloride had a viscosity of 1.32 centipoises Ostwald atfor appearance and the volume of filtrate in the filter ask .wasmeasured.

For treatment in accordance with the invention, the dilution Water added-to the 100 ml. aliquot of sludge 1n the foregoing procedure was dividedinto two equal parts.

To one part thereof, an anionic organic polymer under test was added inamount sufcient to provide a predetermined polymer dosage. The resultingsolution was added to a sludge aliquot and mixed therewith by pouringthe sludge back and forth from one beaker to another for a total of fourtimes. To the remaining por; tion of the dilution water was added thecationic organic polymer. This solution was also poured into the sludgewith mixing by pouring the treated sludge from beaker to beaker asbefore. Note that in Runs l2 and 14 listed below, the foregoing order ofaddition, i.e., anionic polymer first followed by the cationic polymer,was reversed.

The resulting treated sludge was then filtered according to the standardprocedure set forth above. The specific operating conditions and volumeof filtrate obabout 4 percent of its available carboxamide groups tocarboxylic groups and by a viscosity of 50 centipoises Ostwald for anaqueous 0.5 percent by weight solution thereof at a pH of 3 and atemperature of 25 C.

PAA.m(30) refers to a high molecular weight polyacrylamidecharaelerizred indthat 30 percent of the available carboxamide groupswere 7 SMA means a copolymer of styrene and maleic anhydride having anestimated molecular weight of about 4 million.

SSM is a copolymer of mole percent sodium styrene sulfonatc and 15 molepercent methacrylie acid.

9 VBT-AAm means a copolymer containing about 75 mole percent vinylbenzyltrimethylammonium chloride and about 25 mole percent acrylamide.

EXAMPLE 4 Studies of the rate of filtration for untreated and digestedsludge conditioned according to the invention were carried out byfollowing a procedure similar to that set forth in Example 3. In thisexperiment, however, the volumes of the filtrate produced at the end ofseveral time intervals from the initiation of filtration were recorded.Also, in certain runs, the polymer treated sludge was subjected tosevere agitation prior to the filtration step. Improved filtration ratesin such instances are indicative of aggregate stability. Agitationdesignated hereinafter as gentle was accomplished by pouring thesuspension of agglomerated organic solids from one container to anotherfour times. Severe agitation refers to stirring the suspension in a oneliter beaker with a 3 inch wide paddle at 45 r.p.m. for 30 minutes.

Typical filtration rates obtained with the use of combinations ofanionic and cationic organic polymers are summarized in the followingTable 3. In Runs 1 3 the initial undiluted sludge aliquot was 200milliliters and in all others the initial undiluted aliquot wasmilliliters.

Table 3.-Fzltratzon of sewage sludges Dose oi- Dose of- Milliliters ofDry Diluted Filtrate Collected Run Sludge Type Solids Sludge atSpecified Agitaa Lbs./ Lbs./ (gms.) (ml.) Time Intervals tion .AmonioPolymer Ton Catlomc Polymer Ton Dry Dry Solids Solids 10 20 30 Sec. Sec.Sec.

1 Conti-01--.- SPSS(6M) (F.N. 2, 1.0 Activated sludge (blo- 2 230 30Gentle.

Table 2), logically oxidized). 2 Contro1. PEM.) {F.N. 4, 6 do 2 230 130190 213 Do.

Table 2). SPSS(6M) 1.0 PEI B\-.-. 4 do 2 230 140 200 214 Do. 4 CoutrolPEI@ 4 Mixture of raw sludge 10 150 50 72 100 Do.

and activated sludge.

SPSS(6M) .5 PEI E).. d 10 150 70 98 110 Do. 6 Control Epi-DMAOU) 5 16090 135 143 Do. SPSS(6M) Epi-DMA(1).- 5 160 125 145 146 D0. 8 ControlEpi-DMA(10) 10 150 5 10 12 Do.

SPSS(6M) Epi-DMA(10) 10 150 25 55 70 Do. 10 Control.-. PEI@ l0 160 9U117 122 Do. 11 SPSS(6M) 10 160 10U 117 121 D0. 12 Control 10 160 50 S5102 Severe l SPSS (6M) 10 160 90 120 132 Do. 14 Control 10 160 19 30 38Do. 15 PAAJ11(30) 10 160 50 80 100 Do. 16 Control..- 10 160 55 80 98Gentle 17 SPSS(6M) 10 160 90 115 120 D0. 18 Control." 10 160 20 32 40Severe 19 SPSS(6M) 10 160 62 90 106 D0.

10 160 20 30 35 Gentle 10 160 12 16 20 Severe SPSS(6M) 10 160 55 73 103Do.

Footnotes to Table 3; as employed in the present specication:

10 Epi-DMA means the reaction product of epichlorohydrin anddimethylamine in a 1:1 molar ratio and 13 percent by Weight oftetraethylenepentamine.

EXAMPLE The following is illustrative of the application of theinvention to the ltration of anaerobically digested sludge on a plantscale basis. Sludge containing approximately percent solids on a dryWeight basis was pumped into a 150 gallon chemical conditioning tank ata rate of from 10 to 30 gallons per minute. The tank was equipped withthree paddle mixers which operated at r.p.m. on a horizontal axis. Thesludge entering at one end of the tank was treated first with 0.5 poundper ton of dry solids of sodium polystyrene sulfonate having a molecularweight of approximately 4.5 million [SPSS(4.5M)]. The polymer was madeup in the form of a stock solution containing 1 percent by weightpolymer solids and added to the sludge with 6 to 10 gallons of dilutionwater per minute. Thereafter, as the sludge proceeded through theconditioning tank, it was treated with 3.5 pounds per ton of sludge drysolids of polyvinylbenzyltrimethylammonium chloride (VBT) which was madeup in the form of an aqueous solution containing 10 percent by Weight ofthe polymer and likewise added to the sludge with 6 to 10 gallons ofdilution water per minute.

From the conditioning tank the sludge overowed into a sludge pan which,when lled to the proper depth, presented the sludge to rotary vacuumdrum filters. The drum filters were 3 feet in diameter and 6 feet long.They were covered with 32 by 32 mesh saran cloth ilters. The iilterswere operated at a reduced pressure of to 25 inches of mercury.

In a separate but similar operation, the sewage sludge was treated witha conventional system of ferrie chloride and calcium oxide.

The results of the above operations involving the vacuum filtration of adigested sludge on a plant scale basis are set forth in the followingtable.

11 Polyamine X refers to a commercial cationic amine condensationpolymer available as Nalco 600 polyarnne.

12 Polyamine Y refers to commercial catonie amine condensation polymeravailable as Kymene 66 polyamine.

Table 4.-Vacuum filtration of digested sludge plant trial, instantaneousdata (Sludge Treatment) Operation Specifications I F6013/ CaO VET/SPSS 0EXAMPLE 6 Further filtration experiments were carried out with anothermunicipal digested sewage sludge containing 5 percent by Weight solidson a dry basis. 10D milliliter aliquots of this sludge were diluted to1,000 milliliters and subsequently settled for 30 minutes lto simulateelutriation of the sludge. This was done to remove yammonia. alkalinityfrom the sludge. The solids, which settled to a volume of approximatelyto 180 milliliters, were subsequently iiltered after addition ofsuicient dilution Water made up a ltotal volume of 200 milliliters.Separate aliquots of the sludge were treated at various stages duringthe elutriation and iiltration oper-ations with various anionic polymersemployed sequentially Wit-h a cationic polymer. Two of the anionicpolymers used (SPSS(6M) lon a dry basis. n an as received condition,were mixed with suicient poly- 15 and PAAm(30)) and the cationic polymer(PEIm) are described inthe iootnotes to Table 2. A third anionic polymerused was a high molecular Weight sulfonated polyvinyltoluene (SPVT).

The dewatering rates of the underflow solids (settled solids) weredetermined at several time intervals after initiation of the filtrationoperation. Filtration was `accomplished in a Buchner funnelsubstantially according Vto the procedure employed in Example 4.

The yresults of these experiments are reported in the following Tablewherein the polymer doses and stage at which the polymer was introducedinto the sludge are indicated along with the observed ltrate volumes inmilliliters at specied time intervals. i

The results shown in Runs l2, 4, 7, 9 and 11 as compared respectively toRuns 1, 3, 5, 8 and 10 illustrate the mer treating solutions and/oradditional water to make up a total Ivolurne of 1,000 milliliters Withthe simult-aneous achievement of predetermined polymer dosage. Anionicpolymers employed were a high molecular weight sodium polystyrenesulfonate (SPSS(6M)) and a high molecular Weight partially hydrolyzedpolyacryl amide (PAAm()). The cationic polymers were polyethylenimine(PEIm) `and vinylbenzyl trimethylammonium chloride (VBT). These polymerswere constituted substantially -as described for the correspondingsymbols in the footnotes to Table 2.

When the treatment consisted of one polymer, the sludge was stirredgently -ater dilution with the polymer solution for ve minutes. Dualpolymer treatments were accomplished by gentle stirring for threeminutes after the addition -of each of -the polymer solutions.

the filtration operations were conducted.

Table 6.-Plant trial Table 5 Polymer Treatment Volume of Filtrate`Collected at Specified Time Interval Run No. During Elutriation JustPrior to Filtration 1o 2o so so Anionic Cationio Anionic Cationic Sec.Sec. Sec. See.

160 176 182 e5 7s 115 130 140 46 65 su 21bs. SPss(6M} 1Go iso 190 16517e 18s 2113s. sPSSM) es 94 2lb SPV 160 lso 19o 60 es 94 100 12o 16o2110s. PAAm 3o) 50 se 72 importance of adding the cationic polymer tothe sludge 35 Table 7 just prior to filtration. A comparison of Runs 5and 6 indicates good results are obtained regardless of when the PolymerAdded (um/t0n Solids dry anionic polymer is introduced in the system tobe filtered. asis S m* Based on the foregoing experiments a plant trialwas Run I/Aij carried out wherein the SPSS(6M) polymer was incor- 40SPSS i PAAm 30 PEI@ VBT porated into the sludge in an elutriation stepfollowed by treatment of the elutriated and settled solids (underflow(COHUOD gone solids) with the PEI@ polymer just prior to filtration. of?For purposes of comparison, additional `runs were made n N23 onelutriated sludge conditioned ffor filtrationby treat- QG ment of thesludge with 1) ferrie chloride and lime and (2) with PEI@ polymer alone.The results obtained 11:4 are set forth below along with the conditionsunder which Elutriation Run A Run B Run C Chemical treatment SPSS (6M)None None Dose, lbs/ton of solids, dry ham 1.5-. Sludge solids, wt.percent 5.1-. 5.4- 5,4 Vol. wash Water/vol. sludge 8/1.. 8/1- 8/1.Overflow solids, wt. percent 0 033% 0.6% 0.6%. Underflow solids, Wt.percent 8%. Filtration of Under Under-flow Solids:

Chemical treatmentl PEI( PEI( FeCla/CaO.

Dose,1bs./ton of solids, dry basis 5 8 (S0/250.

Sludge flow, gaL/min 40. 27 27.

Dilution water flow, gaL/min 10 20 2,

Filter cake yield, lbs./ft.2/hr 17. 7..-.. 4.

1 These chemicals were added to the sludge stream with gentle agitationjust upstream from the lter.

EXAMPLE 7 The experiments described below demonstrate the fapplicabilityof the conditioning process of the invention t0 vva thickening processfor nonfree settling suspensions of yagglomer-ated organic solids. 'inthese experiments was a digested municipal sludge rIlie suspensionthickened ultimately diluted to about 0.5 percent by weight solids 900milliliter aliquots `of ythe sludge, in

pension of predominately organic, hydrophilic solids not susceptible ofconditioning for dewatering with an anionic organic polymer as the soletreating agent, which solids are in an agglomerated condition and at Iaconcentration whereby the solids are essentially non-free settling,which process comprises adding to the suspension with mixing awater-soluble, cationic, nitrogenous organic polymer, said polymer beingemployed in an amount suiiicient to promote dewatering of thesuspension; the improvement which consists in adding to the suspensionprior to the addition of the nitrogenous organic polymer, and withintervening mixing, a water-soluble, -high molecular weight, anionicorganic polymer characterized by a molecular weight of at least 0.5million 'and as having a polyalkane backbone with a plurality of ionicsubstituent groups selected from the lgroup consisting of carboxylic andsulfonic acid groups iand alkali metal salts of the acid groups; thetotal water of dilution added with the polymers being maintained withinythe range from about 0.()1(S) to about 1.0(S) volumes of water pervolume of liquid aqueous suspension, wherein S equals the parts ofsolids, on a` dry basis, per l() parts of aqueous suspension and saidanionic organic polymer being added to the aqueous suspension in a smallamount, relative to the amount of nitrogenous organic polymer used, suchamount of anionic polymer being eiective to enhance the dewatering rateof the treated aqueous suspension.

2. In a process -for dewatering a liquid, aqueous suspension ofpredominantly iorgani-c, hydrophilic solids, which solids notsusceptible of conditioning for dewatering with an anionic organicpolymer as the sole treating agent are in an agglomerated condition andata concentration whereby the solids are essentially non-free settling,which process comprises adding to the suspension with mixing a diluteaqueous solution of a water-soluble, cationic, nitrogeneous organicpolymer, said polymer being employed in an amount sufficient to promotedewatering of the suspension; the improvement which consists in addingto the suspension prior to the addition of the nitrogenous organicpolymer, and with intervening mixing, a dilute aqueous `solution of awater-soluble, high molecular weight, anionic organic polymercharacterized by a molecular weight of at least 0.5 million and aslhaving a polyalkane backbone with a plurality of ionic substituentgroups selected from the group consisting of carboxylic and sulfonicacid groups and alkali metal salts of the acid groups; the total wateror dilution added with the polymers being maintained within the rangefrom about 0.01(S) to about 1.0(S) volumes of water per volume of liquidaqueous suspension, wherein S equals the parts of solids, on a drybasis, per 100 pants of aqueous suspension and said anionic organicpolymers being added to the aqueous suspension in a small amount,relative to the 'amount of nitrogenous organic polymer used, such amountof anionic polymer being effective to enhance the dewatering rate of thetre-ated .aqueous suspension.

3. A method as in claim 2 wherein the anionic organic polymer isincorporated into the aqueous suspension during clarilication of a wastestream to produce said suspension as a solids underflow.

4. In a process for dewatering a sewage sludge not susceptible ofconditioning for dewatering with an anionic organic polymer as the soletreating agent which comprises adding lto the sludge, with mixing adilute aqueous solution of a water-soluble, cationic, nitrogenousorganic polymer, said polymer being employed in an amount suiiicient topromote dewatering of the sludge; the improvement which consists in.adding t-o the sludge prior to the addition of the nitrogenous organicpolymer, and with intervening mixing, a dilute aqueous solution of awater-soluble, high molecular weight, anionic organic polymercharacterized -by a molecular weight of at least 0.5 million and ashaving a poly-alkane backbone with a plurality of ionic substituentgroups selected from the group consisting of carboxylic land sulfonicacid groups and alkali metal salts of the acid groups; the total waterof dilution added with the polymers being maintained within the rangefrom about 0.01(S) to about 1.0(S) volumes of water per volume of sewagesludge, wherein S equals the parts of solids, on a dry basis, per partsof sewage sludge and said anionic organic polymer being added to thesludge in a small amount, relative to the .amount of nitrogenous organicpolymer used, such amount being effective to enhance the dewatering rateof the treated sludge.

5. A method as in claim 4 wherein the anionic organic polymer employedis a high molecular weight, alkali metal polystyrene sulfonate. 6. Amethod as in claim 4 wherein the anionic organic polymer employed ispartially hydrolyzed polyacrylamide having at least 4 percent of Iitscarboxamide moities hydrolyzed to alkali metal carboxylate groups.

7. In a process for dewatering -a sewage sludge not susceptible ofconditioning for dewatering with an anionic organic polymer .as Ithesole treating agent comprising adding .to the sludge, with mixing, adilute aqueous solution of a water-soluble, cationic, nitrogenousorganic polymer represented by the formula:

wherein R is selected from the group consisting of hydrogen yand methyland n is :at least 400, inclusive, of hydrochloric, sulfuric,phosphoric, carbonite, and acetic acid salts of the foregoing polymers,said polymer being employed in an amount sufficient to promotedewatering of the sludge; the improvement which consists in adding tothe sludge prior to the addition of the nitrogenous organic polymer, andwith intervening mixing suiticient to uniformly distribute the polymerthroughout the sludge, a dilute aqueous solution of a water-soluble,high molecular weight, anionic organic polymer characterized by =amolecular weight of at least 0.5 million and as having -a polyalkanebackbone with a plurality of ionic substituent groups selected strom thegroup consisting of carboxylic and sulfonic acid groups and alkali metalsalts of the acid groups; the total water of dilution added with thepolymers being maintained within the range from about 0.01(S) to about1.0(S) volumes of water per volume of sewage sludge, wherein S equalsthe parts of solids, on a dry basis, per 100 parts of sewage sludge andsaid anionic organic polymer being added to the sludge in -a smallamount, relative to the lamount of nitrogenous organic polymer used,such amount being eiiective to enhance the dewatering rate of thetreated sludge.

8. In a process for dewatering a liquid, aqueous suspension ofpredominately organic, hydrophilic solids not susceptible ofconditioning for dewatering with an anionic organic polymer as the soletreating agent, which sol-ids are in an agglomerated condition and at aconcentration whereby the solids are essentially non-free settling,

which process comprises adding -to the suspension, with mixing, a diluteaqueous solution of a water-soluble, cationic, nitrogenous organicpolymer having a plurality of nitrogenous vgroups selected from thegroup consisting of amino, imino and quaternary nitrogen groups, saidpolymers being employed in lan :amount suicient to promotie dewateringof the sludge, and there-after separating the suspension into solid andliquid phases by filtration; the improvement which consists in adding tothe suspension prior to the addition of the nitrogenous organic polymer,and with intervening mixing suiiicient to uniformly distribute thepolymer throughout the suspension, a dilute aqueous solution of awater-soluble, high molecular weight, anionic organic polymercharacterized by a molecular weight of at least 0.5 million and ashaving a polyalkane backbone with a plurality of ionic ysubstituentgroups selected from the group consisting of carboxylic and sulfonicacid groups and alkali metal salts of the acid groups; the total amountof -the anionic polymer employed being within the range from about 0.01to about 2.5 pounds per ton of organic solids on a dry basis and thetotal water of dilution added with the polymers being maintained withinthe range from about 0.01 (S) to about 1.0(S) volumes of water pervolume of aqueous suspension, wherein S equals the parts of solids, on adry basis, per 100 parts of aqueous suspension, said anionic vorganicpolymer being added to the suspension in a small amount, relative to theamount of nitrogenous organic polymer used, such amount of anionicpolymer ,being effective to enhance the dewatering rate of thesuspension.

9. In a process for dewatering a liquid, aqueous suspension ofpredominantly organic, hydrophilic solids not susceptible ofconditioning for dewatering with an anionic organic polymer as the soletreating agent, whiih solids are in an agglomerated condition and at aconcentration whereby the solids are essentially non-free settling,which process comprises `adding to the suspension, with mixing, a diluteaqueous solution of a water-soluble, cationic, nitrogenous organicpolymer having a plurality of nitrogenous groups selected from the groupconsisting of amino, imino and quaternary nitrogen groups, said polymerbeing employed in an amount sufiicient to promote dewatering of thesludge, and thereafter separating the suspension into solid and liquidphases by introducing the polymer treated suspension into a quiescentzone for a detention period of at least 30 minutes whereby the solidsare thickened by the technique of compaction; the improvement whichconsists in adding to the suspension prior to the addition of thenitrogenous organic polymer, and

with intervening mixing suflicient to uniformly distribute the polymerthroughout the suspension, a dilute aqueous solution of water-soluble,high molecular weight, anionic organic polymer characterized by amolecular weight of at least 0.5 million and as having a polyalkanebackbone with a plurality of ionic substituent groups selected from thegroup consisting of carboxylic and sulfonic acid groups :and alkalimetal salts of the acid groups, the total amount of the anionic polymeremployed being within the range from about 0.5 to about 10 pounds perton of organic solids on a dry basis, and the total water of dilutionadded with the polymers being maintained within the range from about OOltS) to about 1.0(S) volumes of water per volume of aqueous suspension,wherein S equals the parts of solids, on a dry gasis, per 100 parts ofaqueous suspension, said anionic organic polymer being added to thesuspension in a small amount, relative to the amount of ntrogenous`organic polymer used, said amount of anionic polymer being sucient toenhance the dewatering rate of the suspension.

References Cited bythe Examiner UNITED STATES PATENTS 3,142,638 7/1964Blaisdell et al 210-52 FOREIGN PATENTS 826,770 1/ 1960 Great Britain.

MORRIS O. WOLK, Primary Examiner.

M. E. ROGERS, Assistant Examiner.

1. IN A PROCESS FOR DEWATERING A LIQUID, AQUEOUS SUSPENSION OFPREDOMINATELY ORGANIC, HYDROPHILIC SOLIDS NOT SUSCEPTIBLE OFCONDITIONING FOR DEWATERING WITH AN ANIONIC ORGANIC POLYMER AS THE SOLETREATING AGENT, WHICH SOLIDS ARE IN AN AGGLOMERATED CONDITION AND AT ACONCENTRATION WHEREBY THE SOLIDS ARE ESSENTIALLY NON-FREE SETTLING,WHICH PROCESS COMPRISES ADDING TO THE SUSPENSION WITH MIXING AWATER-SOLUBLE, CATIONIC, NITROGENOUS ORGANIC POLYMER, SAID POOLYMERBEING EMPLOYED IN AN AMOUNT SUFFICIENT TO PROMOTE DEWATERING OF THESUSPENSION; THE IMPROVEMENT WHICH CONSISTS IN ADDING TO THE SUSPENSIONPRIOR TO THE ADDITION OF THE NITROGENOUS ORGANIC POLYMER, AND WITHINTERVENING MIXING, A WATER-SOLUBLE, HIGH MOLECULAR WEIGHT, ANIONICORGANIC POLYMER CHARACTERIZED BY A MOLECULAR WEIGHT OF AT LEAST 0.5MILLION AND AS HAVING A POLYALKANE BACKBONE WITH A PLURALITY OF IONICSUBSTITUENT GROUPS SELECTED FROM THE GROUP CONSISTING OF CARBOXYLIC ANDSULFONIC ACID GROUPS AND ALKALI METAL SALTS OF THE ACID GROUPS; THETOTAL WATER OF DILUTION ADDED WITH THE POLYMERS BEING MAINTAINED WITHINTHE RANGE FROM ABOUT 0.01(S) TO ABOUT 1.0(S) VOLUMES OF WATER PER VOLUMEOF LIQUID AQUEOUS SUSPENSION, WHEREIN S EQUALS THE PARTS OF SOLIDS, ON ADRY BASIS, PER 100 PARTS OF AQUEOUS SUSPENSION AND SAID ANIONIC ORGANICPOLYMER BEING ADDED TO THE AQUEOUS SUSPENSION IN A SMALL AMOUNT,RELATIVE TO THE AMOUNT OF NITROGENOUS ORGANIC POLYMER USED, SUCH AMOUNTOF ANIONIC POLYMER BEING EFFECTIVE TO ENHANCE THE DEWATERING RATE OF THETREATED AQUEOUS SUSPENSION.